Adcom GFA-585 Blown 20w 4.7ohm on soft start board - diyAudio
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Old 26th June 2007, 05:44 AM   #1
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Default Adcom GFA-585 Blown 20w 4.7ohm on soft start board

I blew the 3 of the 4 speaker out fuses on my amp with my Polk SDA speakers when hooking it up blindly. I realized my mistake, but wanted to finish my listening session. So I stuck 2 15a, and a 20a fuse in. Fixed the speaker wiring. Turned it on, and poof. I looked for damage, and found none. So I busted out my mm found power on the soft start board. Unplugged it. Started testing resistors. The big 20watt 4.7ohm was open. Took a hammer to it. Found the burnt spot. I am scared to just replace the resistor( I have 2 10ohm 10watt resistors). I don't want to blow anything else. What caused the resistor to blow after installing the speaker fuses?
Should I raplace the rail fuses, The 20watt resistor, and give it a go with 5ampers instead of 10ampers?
Thanks Ben
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Old 26th June 2007, 05:59 AM   #2
lgreen is offline lgreen  United States
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Default amp

Saw your other thread.

1. The soft start resistors blew? I would not blame this on the Polk's cause I know you were not cranking the amp during the soft start cycle, were you????

Typically AC or DC power is shunted through soft start resistors which have a much lower power rating than required. Cause the time is short you usually get away with it. See Elliot Project #39. Do the math and see. Perhaps they just wore out and you need to replace them?

2. No need to start a new thread, people here get what you are talking about.

3. I owned an Adcom 555 II and drove Martin Logan Sequel II's (2 ohm) in bridge mode, never had a problem...heat sinks got really hot. Great amp, might want to bring yours in for a tune-up.
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Old 26th June 2007, 06:25 AM   #3
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Thanks for your reply. I blew the Amps output fuses when I shorted the speaker connector. Then I replaced the fuses after blowing them with ones I had on hand. When I fired it up with no input the resistor burnt. I don't want to fry my amp, and I can't afford to get it serviced. Also I don't trust any of the service shops in this area(Ft Lauderdale) with a complicated amp like mine.

Ben
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Old 26th June 2007, 06:41 AM   #4
lgreen is offline lgreen  United States
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Default Re: Adcom GFA-585 Blown 20w 4.7ohm on soft start board

Quote:
Originally posted by ben62670
I blew the 3 of the 4 speaker out fuses on my amp with my Polk SDA speakers ***
If you have Polk SDA speakers you can afford to have that amp looked at!
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Old 26th June 2007, 01:59 PM   #5
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I have 2 kids, and little income due to health reasons. I have had the SDA's
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Old 26th June 2007, 03:56 PM   #6
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Quote:
Originally posted by ben62670
Thanks for your reply. I blew the Amps output fuses when I shorted the speaker connector. Then I replaced the fuses after blowing them with ones I had on hand. When I fired it up with no input the resistor burnt.
Ben
Hi Ben,

You most likely toasted your output transistors when the original fuses blew. Unfortunately the fuses are pretty slow and don't blow cleanly, so when they do blow allot of times it's after the damage has been done.

When you turned it on with the 20 amp fuse, the fuse was to big and allowed whatever is shorted to take out the 4.7 ohm, which is in series with the power.

Unfortunately it sounds like a trip to shop is required. If Adcom is still around (I don't follow these things anymore) A call to their service department might give you an idea as to what to expect cost wise. Local service can be a roll of the dice if they don't specialize in audio gear and do a fair number of these and similar amps on a regular basis.

Regards Mike.
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Old 26th June 2007, 04:33 PM   #7
KBK is offline KBK  Canada
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yes. at a minimum, the one channel is blown. About $100-125 for the repair. The big issue is the matched output transistors....or..potentially..any OTHER parts you blew up when you put the 20 amp fuse in there.

It needs to go to a experienced technician who is well versed in Adcom repair,and has matched sets of output transistors ready to go. Otherwise, the matching process can be quite expensive, as many have to be bought to match up the nessessary sets. Thus endeth my personal experience with that particular model of adcom.
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Old 26th June 2007, 05:40 PM   #8
anatech is offline anatech  Canada
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Hi Ben,
Mike is correct. Adcom still exists but was bought and moved. They are not really familiar with the older product. Find an old (good) Adcom warranty shop.

You will require two sets of 5 matched outputs for that channel plus driver transistors and some resistors. You can not use five of each type without matching. ECG and NTE parts can not be used either.

That channel will require to be stripped down and cleaned. New grease and insulators and it should be at least as good as new.

Hi Ken,
Quote:
Otherwise, the matching process can be quite expensive, as many have to be bought to match up the necessary sets.
You know it. Also, the repair will be more than $150 USD I'm afraid. You got a smoking deal.

So Ben, the most important thing will be to get your amp into the hands of a really good technician. It's the least expensive route for repair.

-Chris
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Old 26th June 2007, 07:07 PM   #9
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Thanks for the info guys. I take it you are talking about the output transistors on the heat sinks.

Thanks again
Ben
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Old 26th June 2007, 08:19 PM   #10
anatech is offline anatech  Canada
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Hi Ben,
Yes, the outputs.

I am sorry to hear of your problem. With any luck, a rectifier went instead (unlikely, but cheaper to repair).

-Chris
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